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Homework Help: Increase effective load resistance?

  1. Nov 7, 2014 #1
    I am working on my graduation design project, and I am responsible for making the power supply circuit. It is a lower power source, and acts like a current source that can supply current in the range of 30 mA to 130 mA.

    My power supply circuit has a source impedance of 85 ohm. Our load, a microcontroller, can be modelled by a load between 15 ohm to about 200 ohm depending on current draw. At a load ~15 ohms, I only get about ~300 mV at the output. Is there a way to make my load seem larger so voltage division works in my favor?

    The two quick methods I thought don't seem to work to well:
    1. Unity feedback buffer. This requires an op amp, which needs 15 V biasing rails. Since I am the power supply circuit already at a low voltage, I cannot properly bias the amplifier.
    2. I can build more of my power supply circuit and put them in parallel to decrease the source impedance, but this can get expensive and may exceed our project's budget.

    Thanks for all the suggestions in advance!
  2. jcsd
  3. Nov 7, 2014 #2


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    Can you say more about the specification for this power supply circuit? Is it supplying Vdd for the microcontroller (uC)? Or is it supplying power to some other circuit controlled by the uC?

    To say that it is a current source cannot be correct if it is supplying Vdd to the uC. uCs require a Vdd that is typically a voltage with a fairly tight tolerance of +/-10% or so...
  4. Nov 8, 2014 #3
    The power supply circuit is essentially a secondary winding of a transformer, used for power harvesting. It steps down a power line's 500 A down to the reasonable mA range for our electronic circuits. Since the output current of my supply circuit is very dependent on the primary's current, I would model my power supply more as a current source than a voltage source.

    This power supply circuit is suppose to supply Vdd for the microcontroller. My group member working on the uC is using an Arduino and estimates the uC may draw a max 0.25 A at 7 V. This translates to a 28 ohm load during max power requirements. I was thinking that the impedance will convert my output current into an appropriate voltage to supply the uC. However, the 28 ohms is too low compared to the output impedance of my circuit, so I cannot generate enough voltage.

    Of course, I can always tell the uC group members about the limited power budget that they have, so they cannot draw more than x amount of current at 7V.
  5. Nov 8, 2014 #4


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    Sorry, but your post makes no sense at all.

    What powerline is running at 500A. That's nonsense, or misinformation.

    And no Arduino uC runs with a Vdd of 7V.

    Could you please check your facts and repost?
  6. Nov 9, 2014 #5
    Hi berkeman,

    I am talking about distribution power lines, operating at around 12.5 kV. I believe 500 A (in the hundreds of Amps at least) in these is reasonable. My group project is about harvesting some energy from these power lines to power a uC to record some data for the electric utility.

    I believe my teammates is using the Arduino Uno for the uC: http://arduino.cc/en/Main/arduinoBoardUno. It has a recommended input voltage of 7 - 12 V, the lower of which is what I had given you for Vdd.
  7. Nov 9, 2014 #6


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    Well if you are doing a device for the power company, I suppose it is okay for you to harvest power from the distribution lines. In general it is illegal to harvest energy from the distribution lines (outside of metered service), but the power company is the one using the final device.

    The voltage spec 7-12V is the input spec for the voltage regulator on the arduino board, not for the uC and its circuitry itself. That would either be 5V or 3.3V. You lose efficiency (waste precious harvested power) if you use a board that uses a linear regulator to make the final Vdd voltages. The best you can do in that case is put in the lowest allowed input voltage (7V).

    To size the input inductor (that harvests from the magnetic field near the distribution lines), you need to start with the power required by the board (7V * Imax), then work backward through the efficiency of the switching power supply that you put between the input inductor and the Arduino board.
  8. Nov 12, 2014 #7


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    Who is responsible for specifying the transformer? It sounds like someone else has told you to use a specific transformer? Got any data on it?
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